B
Ben C
Guest
On 2008-04-29, Peter Cole <[email protected]> wrote:
> Ben C wrote:
>> On 2008-04-28, Peter Cole <[email protected]> wrote:
>
>> Well my hypothesis (which perhaps you don't regard as plausible
is
>> that this bedding in reduces the bending moment on the spoke.
>>
>> It doesn't matter whether the spoke bends or the hub deforms. The result
>> is a spoke closer to the flange so less moment.
>>
>>> Bottom line is that the deformation, both the majority that comes from
>>> initial tension as well as whatever smaller contribution that may come
>>> from over loading, doesn't predict the direction or degree of bending
>>> stress in the tensioned spoke.
>>
>> I don't understand what you're saying here. Surely for all spokes the
>> effect of hub deformation is going to be to reduce moment on the elbow?
>
> No. Hole deformation will change the elbow support angle slightly. It
> will always enlarge the angle, if the angle was too big to begin with,
> it will increase the moment.
I'm not sure the elbow support angle is the important thing to measure
here, but the perpendicular length of unsupported spoke.
It seems to me that will always get shorter as you are crushing the
fulcrum the spoke is bearing against.
> If the angle was initially too small, the
> angle may be improved, but there still may be a bending stress.
I think there will _always_ be a bending stress unless you've got a
straight pull spoke. So long as it's small enough you're OK though.
>>> As I said before, these threads have gone through an evolution that
>>> began with a denial of the presence of residual stress and have morphed
>>> into a grudging admission with the dismissal for irrelevance. Residual
>>> stresses are present, they may or may not be a factor in any given spoke
>>> failure, but that's impossible to know without knowing all the operating
>>> stresses. No postmortem is going to tell you whether the failure was
>>> caused by a raiser, bending stress, residuals or any combination
>>> thereof.
>>
>> You would expect to see more failures starting in the regions of tensile
>> residual stress (i.e. inside of inbound spokes) if residual stress from
>> manufacturing is a factor.
>>
>> There are other factors of course, but there ought to be a statistical
>> bias.
>
> Who says there is or isn't? If spokes are bowed at the flange and the
> spoke line is not corrected, the bending forces introduced as the spoke
> is tensioned will swamp any residual stresses.
Exactly.
> Those spokes will generally fail at the outside of the bend. There are
> those who refuse to correct a bad spoke line then cite these failures
> as evidence of the irrelevance of residual stress. It's comical,
> really.
Well I don't know who you're talking about there.
>> [...]
>>> These (endless) arguments follow the same pattern. Something is taken
>>> out of context and a straw man is created. Like spoke tension and wheel
>>> strength. The FEA leaves no doubt about what happens when a wheel is
>>> overloaded, it doesn't matter that an unspoked rim will support body
>>> weight without collapsing, the loads we're talking about are the several
>>> g's that happen when the wheel hits a pothole, that's when strength
>>> becomes important. Normally tensioned wheels loose spoke tension at
>>> about the same magnitude of deflection as rim damage starts, looser
>>> tension will mean impact damage will happen earlier. As the loaded area
>>> of the rim becomes slack, the rim looses lateral stiffness while still
>>> under compression and therefore is susceptible to buckling.
>>
>> I follow your account of that. But I reserve judgment on whether a
>> tightly spoked wheel's rim may yield before its spokes go slack.
>
>
> Jobst did an FEA. It's in the book. A similar FEA is here:
> http://www.astounding.org.uk/ian/wheel/3c_rim.html
>
> A loaded spoked wheel doesn't deform into a uniform oval, it develops a
> flat spot. As the length of the flat spot grows (with increasing load)
> the bending stress on the rim increases. When that stress reaches a
> critical value the bend is permanent. The rate at which the flat spot
> grows with load is related to the initial spoke tension.
Yes, I know, and I am familiar with Ian's FEA.
> That's it. That you can ride around on a loosely spoked wheel doesn't
> say anything about what happens when it meets a pothole.
I don't think it's so easy to predict what happens when you meet a
pot-hole.
[...]
>>> When several sources are found that confirm the anodization fatigue
>>> connection, all that is dismissed with talk of anisotropy and
>>> extrusion flaws -- factors (obviously) still there whether anodization
>>> is or is not.
>>
>> That is a much harder one to call. Some of these debates can be well
>> understood with basic mechanics and understanding of stress/strain and
>> S-N curves, which are simplified macroscopic views of how materials
>> behave.
>>
>> But the effects of anisotropy and anodization on fatigue life is getting
>> much deeper into the structure of metals. Yes we know in the most
>> general terms they both can be factors but that's a long way from
>> understanding it enough to know how to apply it to bicycle rims.
>
> Nonsense. The "defenses" of anodizing boil down to an assertion that the
> rim extrusions are so crappy that it doesn't matter if they take an
> additional reliability hit from anodizing.
Well, you can put it like that, but that could be misleading.
Designing something like a rim is an optimization problem with
constraints on things like strength, fatigue life, stiffness, weight,
cost and parameters like choice of alloy, shape of extrusion, whether
you anodize.
It won't necessarily be possible to get on the limits of all the
constraints: there may be more constraints than parameters.
It's rather like the recent discussion of forks: for aluminium forks the
fatigue constraint probably dominates, for steel forks, strength, or
resistance to crumpling or getting dented.
So, it may be that the best way to make a rim to satisfy all the other
requirements leaves it just on the limit of fatigue due to anisotropy.
If that were the case then it wouldn't do any harm to anodize it.
Understanding how to design something like that and making educated
guesses which are likely to be the dominant factors _does_ involve
understanding the materials properly.
There is also the evidence that the cracks tend to be oriented on the
path along which the rim was extruded. Now some people have suggested
anodizing could cause a crack to start and then hoop stress or
anisotrophy cause it to propagate on that path. Unforgiven98, on the
other hand, who seems to know what he is talking about, has said that
anodizing and anistropy do not work together ("collude" I think was the
term).
Finally just because anodizing causes fatigue in some applications it
doesn't follow that it does in all. I'm sure there's more too it--
different kinds of anodizing, what sort of aluminium you're using, etc.
> If you have to derate spoke tension below what all the other wheel
> components can tolerate just to prevent socket cracking then you've
> just got weak sockets. Of course if you don't understand the benefit
> of high spoke tension you don't think you've given anything up.
I think there's a similar sort of balance there too.
> Ben C wrote:
>> On 2008-04-28, Peter Cole <[email protected]> wrote:
>
>> Well my hypothesis (which perhaps you don't regard as plausible
is>> that this bedding in reduces the bending moment on the spoke.
>>
>> It doesn't matter whether the spoke bends or the hub deforms. The result
>> is a spoke closer to the flange so less moment.
>>
>>> Bottom line is that the deformation, both the majority that comes from
>>> initial tension as well as whatever smaller contribution that may come
>>> from over loading, doesn't predict the direction or degree of bending
>>> stress in the tensioned spoke.
>>
>> I don't understand what you're saying here. Surely for all spokes the
>> effect of hub deformation is going to be to reduce moment on the elbow?
>
> No. Hole deformation will change the elbow support angle slightly. It
> will always enlarge the angle, if the angle was too big to begin with,
> it will increase the moment.
I'm not sure the elbow support angle is the important thing to measure
here, but the perpendicular length of unsupported spoke.
It seems to me that will always get shorter as you are crushing the
fulcrum the spoke is bearing against.
> If the angle was initially too small, the
> angle may be improved, but there still may be a bending stress.
I think there will _always_ be a bending stress unless you've got a
straight pull spoke. So long as it's small enough you're OK though.
>>> As I said before, these threads have gone through an evolution that
>>> began with a denial of the presence of residual stress and have morphed
>>> into a grudging admission with the dismissal for irrelevance. Residual
>>> stresses are present, they may or may not be a factor in any given spoke
>>> failure, but that's impossible to know without knowing all the operating
>>> stresses. No postmortem is going to tell you whether the failure was
>>> caused by a raiser, bending stress, residuals or any combination
>>> thereof.
>>
>> You would expect to see more failures starting in the regions of tensile
>> residual stress (i.e. inside of inbound spokes) if residual stress from
>> manufacturing is a factor.
>>
>> There are other factors of course, but there ought to be a statistical
>> bias.
>
> Who says there is or isn't? If spokes are bowed at the flange and the
> spoke line is not corrected, the bending forces introduced as the spoke
> is tensioned will swamp any residual stresses.
Exactly.
> Those spokes will generally fail at the outside of the bend. There are
> those who refuse to correct a bad spoke line then cite these failures
> as evidence of the irrelevance of residual stress. It's comical,
> really.
Well I don't know who you're talking about there.
>> [...]
>>> These (endless) arguments follow the same pattern. Something is taken
>>> out of context and a straw man is created. Like spoke tension and wheel
>>> strength. The FEA leaves no doubt about what happens when a wheel is
>>> overloaded, it doesn't matter that an unspoked rim will support body
>>> weight without collapsing, the loads we're talking about are the several
>>> g's that happen when the wheel hits a pothole, that's when strength
>>> becomes important. Normally tensioned wheels loose spoke tension at
>>> about the same magnitude of deflection as rim damage starts, looser
>>> tension will mean impact damage will happen earlier. As the loaded area
>>> of the rim becomes slack, the rim looses lateral stiffness while still
>>> under compression and therefore is susceptible to buckling.
>>
>> I follow your account of that. But I reserve judgment on whether a
>> tightly spoked wheel's rim may yield before its spokes go slack.
>
>
> Jobst did an FEA. It's in the book. A similar FEA is here:
> http://www.astounding.org.uk/ian/wheel/3c_rim.html
>
> A loaded spoked wheel doesn't deform into a uniform oval, it develops a
> flat spot. As the length of the flat spot grows (with increasing load)
> the bending stress on the rim increases. When that stress reaches a
> critical value the bend is permanent. The rate at which the flat spot
> grows with load is related to the initial spoke tension.
Yes, I know, and I am familiar with Ian's FEA.
> That's it. That you can ride around on a loosely spoked wheel doesn't
> say anything about what happens when it meets a pothole.
I don't think it's so easy to predict what happens when you meet a
pot-hole.
[...]
>>> When several sources are found that confirm the anodization fatigue
>>> connection, all that is dismissed with talk of anisotropy and
>>> extrusion flaws -- factors (obviously) still there whether anodization
>>> is or is not.
>>
>> That is a much harder one to call. Some of these debates can be well
>> understood with basic mechanics and understanding of stress/strain and
>> S-N curves, which are simplified macroscopic views of how materials
>> behave.
>>
>> But the effects of anisotropy and anodization on fatigue life is getting
>> much deeper into the structure of metals. Yes we know in the most
>> general terms they both can be factors but that's a long way from
>> understanding it enough to know how to apply it to bicycle rims.
>
> Nonsense. The "defenses" of anodizing boil down to an assertion that the
> rim extrusions are so crappy that it doesn't matter if they take an
> additional reliability hit from anodizing.
Well, you can put it like that, but that could be misleading.
Designing something like a rim is an optimization problem with
constraints on things like strength, fatigue life, stiffness, weight,
cost and parameters like choice of alloy, shape of extrusion, whether
you anodize.
It won't necessarily be possible to get on the limits of all the
constraints: there may be more constraints than parameters.
It's rather like the recent discussion of forks: for aluminium forks the
fatigue constraint probably dominates, for steel forks, strength, or
resistance to crumpling or getting dented.
So, it may be that the best way to make a rim to satisfy all the other
requirements leaves it just on the limit of fatigue due to anisotropy.
If that were the case then it wouldn't do any harm to anodize it.
Understanding how to design something like that and making educated
guesses which are likely to be the dominant factors _does_ involve
understanding the materials properly.
There is also the evidence that the cracks tend to be oriented on the
path along which the rim was extruded. Now some people have suggested
anodizing could cause a crack to start and then hoop stress or
anisotrophy cause it to propagate on that path. Unforgiven98, on the
other hand, who seems to know what he is talking about, has said that
anodizing and anistropy do not work together ("collude" I think was the
term).
Finally just because anodizing causes fatigue in some applications it
doesn't follow that it does in all. I'm sure there's more too it--
different kinds of anodizing, what sort of aluminium you're using, etc.
> If you have to derate spoke tension below what all the other wheel
> components can tolerate just to prevent socket cracking then you've
> just got weak sockets. Of course if you don't understand the benefit
> of high spoke tension you don't think you've given anything up.
I think there's a similar sort of balance there too.
